Graphenic carbon nanoparticles having a low polyaromatic hydrocarbon concentration and processes of making same

ABSTRACT

Provided are graphene nanosheets having a polyaromatic hydrocarbon concentration of less than about 0.7% by weight. Also provided are Graphene nanosheets having a polyaromatic hydrocarbon concentration of about 0.01% to about 0.5%.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. Ser. No. 17/866,560filed on Jul. 18, 2022, that is a continuation of U.S. Ser. No.16/484,885 filed on Aug. 9, 2019 (issued as U.S. Pat. No. 11,420,873 onAug. 23, 2022) that is a 35 USC 371 national stage entry ofPCT/CA2018/050145 filed on Feb. 8, 2018, and which claims priority toU.S. 62/457,472 filed on Feb. 10, 2017. These documents are herebyincorporated by reference in their entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to the field of graphenic carbonnanoparticles and more particularly to graphene nanosheets having areduced content in polyaromatic hydrocarbons (PAHs) and processesthereof.

BACKGROUND OF THE DISCLOSURE

Commercially available graphene can be split into 3 categories:single-layer graphene from chemical vapour deposition (CVD) on asubstrate, multi-layer graphene from graphite exfoliation and few-layergraphene nanosheets produced using a plasma torch. While CVD graphenepossesses the qualities of true single-layer graphene, it will likelynever be produced in quantities necessary for bulk applications.Exfoliated multi-layer graphene, while being available in bulkquantities suitable for energy storage, filler and conductive inkapplications, does not possess the specifications or spectral signatureof mono-layer graphene nor can it approach the electrical conductivityvalues expected for mono-layer graphene. Few-layer and multilayergraphene, also referred to herein as graphene nanosheets, are a focus ofthe present disclosure.

Few-layer graphene nanosheets can be produced in bulk quantities andwith a signature (Raman spectra and specific surface area) similar tothat of monolayer graphene by plasma torch processes such as describedin U.S. Pat. Nos. 8,486,363, 8,486,364 and 9,221,688, U.S. provisionalapplication No. U.S. 62/437,057 and PCT application no. WO 2015189643A1, which are incorporated herein by reference in their entirety.However, the production of graphene nanosheets by plasma processes leadsto the formation of polyaromatic hydrocarbons (PAHs) as a by-product,usually with a concentration in the range of about 0.1 to about 2% byweight. In such processes, PAHs form on the surface of the few-layergraphene nanosheets.

PAHs are undesired compounds present on carbon-based powders producedfrom the pyrolysis of gaseous hydrocarbon precursors or when a mixtureof hydrogen precursor and carbon precursor are simultaneously presentduring the production of carbon-based powders. PAHs encompass manycompounds composed primarily of carbon and hydrogen (C_(X)H_(Y)) andwhere carbon is mostly arranged in aromatic ring configuration with sp²hybridization. PAHs can also contain small fractions of oxygen ornitrogen or other atoms. PAHs can be noxious and carcinogenic as well aspose a serious hazard to humans handling carbon nanoparticles containingPAHs as well as consumers using products that contain PAHs (See Borm PJ, et. al., Formation of PAH-DNA adducts after in vivo and vitroexposure of rats and lung cells to different commercial carbon blacks,Toxicology and Applied Pharmacology, 2005 Jun. 1; 205(2): 157-167.). Asa consequence, regulations exist to limit the fraction of PAHs presentin manufactured carbon powder (as an example, the EU directive2007/19/EC establishes a maximum Benzo(a)pyrene content of 0.25 mg/kg incarbon black). Moreover, the presence of PAH on carbon surfaces can havedetrimental effects on the performance in energy storage applications byblocking small pores and therefore by decreasing the specific surfacearea.

In addition, the Harmonized System (HS), established by the World CustomOrganization (WCO), classifies many PAHs as Category 1B carcinogenic,mutagenic or reprotoxic (CMR) substances. Accordingly the new EuropeanREACH Annex XVII has limited the concentration of PAH in consumerproducts to 0.0001% by weight for (or 1 mg/kg).

Wet chemistry processes to wash or rinse off PAHs from carbon particlesare known. Such processes, such as Soxhlet extraction, generally requirethe use of toxic non-polar solvents such as toluene, since thesolubility of PAHs is very limited. However, such processes involvingtoxic solvents lead to large amounts of waste formed by solventscontaminated with PAHs. Wet-chemistry PAH removal processes thus have anegative environmental impact and add a large cost to the PAH-free endproduct. It is thus highly desirable to develop a simple gas-phase (dry)method to remove PAHs from carbon nanoparticles and graphene nanosheetsand especially plasma-grown graphene nanosheets that is also economicaland does not involve solvent waste. The use of liquid-phase processesalso leads to significant densification of the carbon powder once dried.Such higher density may be detrimental to further processing such asdispersion, for example.

It is thus highly desirable to produce directly, using a plasma process,and without post-processing, graphene nanoplatelets containing very lowlevels of PAHs. Indeed, while it is possible to wash away PAHs using wetchemistry processes such as Soxhelet extraction, this adds much cost tothe final PAH-free graphene material.

THE SUMMARY

The present disclosure relates to graphene nanosheets with low amountsof polyaromatic hydrocarbons. These graphene nanosheets do not requiregoing through a liquid-phase or wet-chemistry process and thus display alower tap density. The present disclosure further relates to processesof making the graphene nanosheets of the present disclosure.

There is provided in an aspect graphene nanosheets having a polyaromatichydrocarbon concentration of less than about 0.7% by weight.

There is provided in another aspect graphene nanosheets having apolyaromatic hydrocarbon concentration of less than about 0.7% by weightand a tap density of less than about 0.08 g/cm³, as measured by ASTMB527-15 standard.

Also provided in another aspect is a process for removing volatileimpurities from graphene nanosheets, comprising heating the graphenenanosheets under a reactive atmosphere, at a temperature of at leastabout 200° C.

In another aspect, there is provided a process for increasing thespecific surface area (B.E.T.) of graphene nanosheets, wherein theprocess comprises heating the graphene nanosheets under oxidativeatmosphere, at a temperature of at least about 200° C.

In another aspect, there is provided a process for dispersing graphenenanosheets in a solvent, wherein the process comprises heating thegraphene nanosheets under oxidative atmosphere, at a temperature of atleast about 200° C. and dispersing the graphene nanosheets in a solvent.

In another aspect, there is provided a process for improving theelectrical conductivity of graphene nanosheets, wherein the processcomprises heating the graphene nanosheets under oxidative atmosphere, ata temperature of at least about 200° C.

There is provided herein in an aspect a plasma process for producinggraphene nanosheets comprising:

-   -   injecting into a thermal zone of a plasma a carbon-containing        substance at a velocity of at least 60 m/s standard temperature        and pressure (STP) to nucleate the graphene nanosheets, and        quenching the graphene nanosheets with a quench gas of no more        than 1000° C., and    -   further heating the graphene nanosheets under reactive        atmosphere, at a temperature of at least about 200° C.

In another aspect, there is provided herein a plasma process forproducing graphene nanosheets comprising:

-   -   injecting into a thermal zone of a plasma a carbon-containing        substance at a velocity of at least 60 m/s STP to nucleate the        graphene nanosheets, and quenching the graphene nanosheets with        a quench gas of no more than 1000° C., thereby producing the        graphene nanosheets with a Raman G/D ratio greater than or equal        to 3 and a 2D/G ratio greater than or equal to 0.8, as measured        using an incident laser wavelength of 514 nm, and    -   further heating the graphene nanosheets under reactive        atmosphere, at a temperature of at least about 200° C.

In a further aspect, there is provided herein a plasma process forproducing graphene nanosheets, comprising:

-   -   injecting into a thermal zone of a plasma a carbon-containing        substance at a velocity of at least 60 m/s STP and at a quench        gas to carbon ratio of at least 75 standard liter per minute        (slpm) of quench gas per mole of carbon injected per minute,        thereby producing the graphene nanosheets, and    -   further heating the graphene nanosheets under reactive        atmosphere, at a temperature of at least about 200° C.

In a further aspect, there is provided herein a plasma process forproducing graphene nanosheets, comprising:

-   -   injecting into a thermal zone of a plasma a carbon-containing        substance at a velocity of at least 60 m/s STP and at a quench        gas to supplied plasma torch power ratio of at least 1.25 slpm        of quench gas per kW of supplied plasma torch power, thereby        producing the graphene nanosheets, and    -   further heating the graphene nanosheets under reactive        atmosphere, at a temperature of at least about 200° C.

In yet another aspect, there is provided herein a plasma process forproducing graphene nanosheets, comprising:

-   -   injecting into a thermal zone of a plasma a carbon-containing        substance, the injecting of the carbon-containing substance        being carried out using a plurality of jets at a velocity of at        least 60 m/s STP and directed such that the injected        carbon-containing substance is distributed radially about a        torch axis and diluted before reaching a quench gas, thereby        producing the graphene nanosheets with a Raman G/D ratio greater        than or equal to 3 and a 2D/G ratio greater than or equal to 0.8        as measured using an incident laser wavelength of 514 nm, and    -   further heating the graphene nanosheets under reactive        atmosphere, at a temperature of at least about 200° C.

Another aspect herein provided is a plasma process for producinggraphene nanosheets, comprising:

-   -   injecting into a thermal zone of a plasma a carbon-containing        substance at a velocity of at least 60 m/s STP and at a quench        gas to supplied plasma torch power ratio of at least 1.25 slpm        of quench gas per kW of supplied plasma torch power, thereby        producing the graphene nanosheets at a rate of at least 120 g/h,        and further heating the graphene nanosheets under reactive        atmosphere, at a temperature of at least about 200° C.

Another aspect herein provided is a plasma process for producinggraphene nanosheets, comprising:

-   -   injecting into a thermal zone of a plasma a carbon-containing        substance, the injecting of the carbon-containing substance        being carried out using a plurality of jets at a velocity of at        least 60 m/s STP and directed such that the injected        carbon-containing substance is distributed radially about a        torch axis and diluted before reaching a quench gas, thereby        producing the graphene nanosheets at a rate of at least 120 g/h,        and    -   further heating the graphene nanosheets under reactive        atmosphere, at a temperature of at least about 200° C.

A further aspect herein provided is a plasma process for producinggraphene nanosheets, comprising:

-   -   injecting into a thermal zone of a plasma a carbon-containing        substance at a velocity of at least 60 m/s, thereby producing        the graphene nanosheets at a rate of at least 2 g/kWh of        supplied plasma torch power, and    -   further heating the graphene nanosheets under reactive        atmosphere, at a temperature of at least about 200° C.

In a further aspect, there is provided herein a plasma process forproducing graphene nanosheets, comprising:

-   -   injecting into a thermal zone of a plasma a carbon-containing        substance at a velocity of at least 60 m/s and with a supplied        plasma torch power greater than 35 kW, thereby producing the        graphene nanosheets at a rate of at least 80 g/h, and    -   further heating the graphene nanosheets under reactive        atmosphere, at a temperature of at least about 200° C.

In a further aspect, there is provided herein a plasma process forproducing graphene nanosheets, comprising:

-   -   injecting into a thermal zone of a plasma natural gas or methane        at a velocity of at least 60 m/s STP to nucleate the graphene        nanosheets, and quenching the graphene nanosheets with a quench        gas, and further    -   heating the graphene nanosheets under reactive atmosphere, at a        temperature of at least about 200° C.

It has been found that the processes described herein are effective forremoving polyaromatic hydrocarbons thus allowing for economical andlarge-scale production of graphene nanosheets that have very low PAHcontent and are safe to handle and to integrate into end-userapplications. Furthermore, the processes described herein are effectivefor cleaning the surface of the graphene nanosheets, increasing theirspecific surface area and improving the ability of electrons to flowfreely along their surfaces. The processes are thus effective forimproving the electrical conductivity properties of the graphenenanosheets.

The products and processes described herein are effective for increasingthe ability of the graphene nanosheets to be dispersed in solvents,thereby increasing their usability and performance in conductiveapplications where percolation at low loadings is advantageous.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings, which represent by way of example only,various embodiments of the disclosure:

FIG. 1A is a top view and FIG. 1B is a cross-sectional view along thelines of A-A of FIG. 1A showing a thermal enclosure (oven) used to carryout the thermal treatment.

FIG. 2A (bottom view) and FIG. 2B (cross sectional view taken along theline 1B-1B of FIG. 1A) show a five (5)-hole shower head-type nozzle usedto inject the carbon-containing substance.

FIG. 3 is a plot of a Raman spectra obtained with an incident wavelengthof 514 nm from a sample produced using a multi-hole injector where, foreach of these injector holes, the injection velocity was greater orequal than 60 m/s STP (standard temperature and pressure) and theinjection angle is 25 degrees with respect to the axis of symmetry ofthe plasma.

FIG. 4 is a plot of Raman spectra obtained with an incident wavelengthof 514 nm from a sample produced using a single-hole injector and lowerinjection velocity (less than 60 m/s STP).

FIG. 5 shows the plasma torch with a multi-hole injector used in example1 and the qualitative flow of the gases including the non-carboncontaining gases and the carbon containing substance.

FIG. 6 shows the plasma torch with a single-hole injector used inexample 2 and the qualitative flow of the gases including the non-carboncontaining gases and the carbon containing substance.

DETAILED DESCRIPTION OF THE DISCLOSURE

The expression “graphene nanosheets” as used herein refers to crumpledgraphene nanosheets having structures comprising one or more stackedlayers of one-atom-thick sheets of sp²-bonded carbon atoms arranged in ahoneycomb lattice. A least a portion of these stacked sheets are curled,curved or buckled, giving them a 3D morphology. Such particles are alsoknown as graphene nanoplatelets (GNP), graphene nanoflakes, crumpledgraphene, few-layer graphene, graphenic carbon particles or simplygraphene. For example, graphene nanosheets can refer to particlescomposed of 10 layers or less and displaying high B.E.T. specificsurface area (≥250 m²/g) as measured by ASTM D 3663-78 standard(Brunauer et al.). The particles have a thickness ranging between 0.5-10nm and widths typically greater than or equal to 50 nm, and thus displaya high aspect ratio of at least 5:1 but typically greater or equal than10:1. The particles, when analyzed using Raman spectroscopy with anincident laser wavelength of 514 nm, display the typical D, G and 2Dbands (located at about 1350 cm⁻¹, 1580 cm⁻¹ 2690 cm⁻¹ respectively) anda G/D ratio greater or equal than 3 (G/D≥3) as well as a 2D/G ratiogreater or equal than 0.8 (2D/G≥0.8). As used herein, the G/D and 2D/Gratios refer to the ratios of the peak intensity of these bands.Graphene nanosheets can for example be made from plasma torch processesas described in U.S. Pat. Nos. 8,486,363, 8,486,364 and 9,221,688 aswell as provisional application No. U.S. 62/437,057. These documents arehereby incorporated by reference in their entirety.

The expression “aspect ratio” as used herein refers to the ratio of thelongest dimension of the graphene particle to the shortest dimension ofthe graphene particle. For example, a graphene particle having anaverage width of 100 nm and an average thickness of 2 nm has an aspectratio of 50:1.

The expression “polyaromatic hydrocarbon”, “PAH” or “PAHs” as usedherein refers to a group of chemicals that are formed during theincomplete burning of coal, oil, gas, wood, garbage, or other organicsubstances, such as tobacco and charbroiled meat. There are more than100 different PAHs. PAHs generally occur as complex mixtures (forexample, as part of combustion products such as soot), not as singlecompounds. They can also be found in substances such as for examplecrude oil, coal, coal tar pitch, creosote, and roofing tar. The list ofPAHs includes but is not limited to Biphenylene, Acenaphthylene,Phenanthrene, Anthracene, Fluoranthene, Pyrene, Xylenes, Napthalene,Benzo(A)Pyrene (BaP), Benzo[E]pyrene (BeP), Benzo[a]anthracene (BaA),Chrysen (CHR), Benzo[b]fluoranthene (BbFA), Benzo[j]fluoranthene (BjFA),Benzo[k]fluoranthene (BkFA), and Dibenzo[a,h]anthracene (DBAhA).

The expression “reactive atmosphere” or “reactive environment” as usedherein refers for example to an oxidative atmosphere or a reducingatmosphere.

The terms “oxidative atmosphere” or “oxidative environment” as usedherein refer to atmospheres containing at least one oxidation agent asdescribed herein.

The expression “oxidation agent” as used herein refers to a gas mixturecomprising, but not limited to: air, oxygen, ozone, peroxides (such ashydrogen peroxide), F₂, CO₂, H₂O, NO₂, Cl₂, or oxidizing acids such asalcohols, sulfuric acid, perchloric acid, persulfates acid, hypohalites(such as sodium hypochlorite), mixtures thereof. The gas mixture canalso comprise a noble gas (such as Ar) or N₂.

The expressions “reducing atmosphere” or “reducing environment” as usedherein refer to atmospheres containing at least one reducing agent asdescribed herein.

The expression “reduction agent” as used herein refers to NH₄, H₂, H₂S,CO, and mixtures thereof.

The concentration of polyaromatic hydrocarbons in a graphene sample canbe determined quantitatively for example by Soxhlet extraction intoluene, followed by analysis using gas chromatography mass spectrometry(GC/MS), as is common for the quantification of Benzo-α-Pyrene (BaP) incarbon black samples. A standard method to quantify polyaromatichydrocarbons in carbon samples is described by the standard ASTMD7771-17, “Standard Test Method for Determination of Benzo-α-Pyrene(BaP) Content in Carbon Black”. While this standard focuses onBenzo-α-Pyrene (BaP), the measurement method can be used for othercompounds of the PAH family. Our concentration in percent PAHs reportedis the sum of all detected PAHs. Our Soxhlet extractions were typicallyonly about 4-6 hours compared with 16 hours for the ASTM standard. TheSoxhlet was set up for high efficiency extraction with rapid fill/draincycles. The eluent was colorless prior to the extraction beingterminated. The extract was not concentrated but analyzed directly byGC/MS and compared with commercially available standard PAH mixtures.The detection limit of this method is of the order of 35-90 ppm PAH(0.0035-0.0090% PAH by weight).

The expression “tap density” as used herein refers to a measurementobtained by mechanically tapping a graduated cylinder containing asample until little further volume change is observed, as described bythe ASTM standard B527-15 “Standard Test Method for Tap Density of MetalPowders and Compounds”. The tapped density is calculated as mass dividedby the final volume of the powder (e.g. g/cm³).

The expression “thermally produced” as used herein refers to graphenenanosheets that were produced by a plasma process. Examples aredescribed in U.S. Pat. Nos. 8,486,363, 8,486,364 and 9,221,688 as wellas provisional application No. U.S. 62/437,057, all of which are herebyincorporated by reference in their entirety.

The “substantially unchanged” as used herein when referring to the tapdensity means that following a thermal reactive treatment describedherein, the tap density of the treated graphene nanosheets will beincreased or decreased by less than about 10%, 9%, 8%, 7%, 6%, 5%, 4%,3%, 2% or 1%.

The expression “carbon-containing substance” as used herein refers to acompound or substance that comprises at least one carbon atom.

The expression “thermal zone” as used herein refers to a thermal zonethat can be generated for example by a quasi-thermal plasma, forexample, a plasma that is close to local thermodynamic equilibrium(LTE), formed by, for example, an inductively coupled plasma torch(ICP), a direct-current plasma torch (DC-plasma), an alternative-currentplasma (AC-plasma) or a microwave plasma torch or any other suitable wayto generate a hot gas in the plasma state. A plasma is close to LTE athigh pressure (typically over 100 torr), where collisions betweenelectrons, ions, neutrals and radicals are frequent.

The term “supplied plasma torch power” as used herein refers to thepower supplied to the plasma torch. The supplied power is greater thanor equal to the power in the plasma as plasma torches are not 100percent efficient at transferring the supplied power to the plasma gas.

The term “quench gas to carbon ratio” as used herein refers to thevolume per unit of time of quench gas, for example standard liter perminute (slpm) of gas injected, for the volume per unit of time (forexample slpm) of a carbon-containing substance, for example acarbon-containing gas injected. The term “quench gas to carbon ratio” asused herein also refers to the volume per unit of time of quench gas tothe number of moles of carbon injected (1 mole of carbon is equal to 12grams of carbon). The “quench gas to carbon ratio” as used herein alsorefers to the mass per unit of time (for example gram per second or gramper minute) of quench gas injected into the reactor to the mass per unitof time (for example gram per second or gram per minute) of acarbon-containing substance.

As used herein, the term “quench gas” refers to and can comprise anynon-carbon containing gas with a high thermal conductivity at STPgreater than or equal to 17.9 milli-Watt per meter per degree Kelvin(the thermal conductivity of Argon at STP; see E. W. Lemmon and R. TJacobsen). The quench gas may for example be composed of argon, helium,hydrogen, nitrogen or any other gas with a thermal conductivity greaterthan or equal to 17.9 mW/m·K, or any mixture of these gases. A personskilled in the art will understand that the thermal conductivity of thegas is determinant for the quench rate of the reactants. The quench gaswill typically be injected close to or inside the plasma torch but canbe injected elsewhere in the reactor as well as in multiple layers ormultiple locations. As used herein, the “quench gas” also refers to asheath gas injected next to the plasma gas in a RF-plasma or DC-plasmatorch and used to protect the torch components from thermal shock anddegradation (see FIGS. 5 and 6 ).

As used herein, all gas volumes and velocities are, unless specifiedotherwise, meant to denote quantities at standard temperature andpressure (STP). The person skilled in the art will readily understandthat these values change at high temperature and high pressureexperienced in the plasma torch.

Terms of degree such as “about” and “approximately” as used herein meana reasonable amount of deviation of the modified term such that the endresult is not significantly changed. These terms of degree should beconstrued as including a deviation of at least ±5% or at least ±10% ofthe modified term if this deviation would not negate the meaning of theword it modifies.

The present disclosure relates to graphene nanosheets having a lowcontent of polyaromatic hydrocarbons without having undergone aprocessing step in the liquid phase, such as a soxhlet extraction. Thesegraphene nanosheets may display a low tap density below about 0.06g/cm³, as described by the ASTM standard B527-15 “Standard Test Methodfor Tap Density of Metal Powders and Compounds”. The PAH concentrationin this material can be less than about 0.3% by weight, less than about0.1% by weight, less than about 0.01% by weight, or less than thedetection limit of the gas chromatography mass spectrometry (GC/MS)apparatus. The graphene nanosheets can feature a Raman G/D ratio greaterthan or equal to about 2, a 2D/G ratio greater than or equal to about0.8 (when measured using an incident laser with a wavelength of 514 nm)and a specific surface area (B.E.T.) of about 250 m²/g or greater. Thetap density of the graphene nanosheets will typically be between about0.03 and about 0.05 g/cm³. In accordance with embodiments of the presentdisclosure, the thermally produced graphene nanosheets may be producedby processes and methods as disclosed for example in U.S. Pat. Nos.8,486,363, 8,486,364 and 9,221,688, which are hereby incorporated hereinby reference in their entirety.

A method to obtain graphene nanosheets comprising a low PAH contentcomprises exposing the graphene nanosheets containing PAHs to a thermaltreatment at a temperature greater than 200° C. or greater than 300° C.in an atmosphere containing a reactive species, for example an oxidativespecies such as oxygen. The duration of this heat treatment underoxidative environment can be applied for a duration of one hour or more.The temperature in the enclosure (e.g. the oven) containing the carbonnanoparticles or graphene can be raised gradually, and the gaseousatmosphere can be a mixture of an inert gas and a reactive species. Forexample, when the reactive species is an oxidative species, the gasmixture can be a mixture of nitrogen and oxygen, a mixture of argon andoxygen, air, a mixture of argon, nitrogen and oxygen, or any othermixture of an oxidative species and inert species. The reactive speciescan also be a reducing species. The pressure in the enclosure can bebelow atmospheric pressure (a partial vacuum), at atmospheric pressureor above atmospheric pressure. For example, the treatment can be carriedout in vacuum or at high pressure in an oxidizing atmosphere (e.g., air,a mixture of oxygen and argon or a mixture or oxygen and nitrogen, orany other gas mixture containing an oxidation agent), such that the PAHor a portion thereof is removed. The graphene nanosheets can besubjected to a sufficient temperature on the order of from about 300° C.to about 500° C. (or higher, such as 500° C. to 650° C.). The heatingcan occur for any time sufficient to achieve the removal of the PAH. Theheating can occur in any type of furnace or other device capable ofsubjecting particulates to heat under a reactive atmosphere andpreferably at atmospheric pressure. The temperature can be from 200° C.to 500° C., such as 290° C. to 500° C., or 400° C. to 500° C.Temperatures above 500° C. may be used, such as 500° C. to 650° C. orfrom 500° C. to 650° C. or higher. A person skilled in the art willappreciate that graphene particles can be oxidized and be burned anddestroyed in oxidative environments at temperatures above 600° C. Aperson skilled in the art will appreciate that exposing the grapheneparticles to higher temperature at lower oxygen concentration can have asimilar effect as exposing the particles to lower temperatures andhigher oxygen concentrations.

The graphene nanosheets resulting from the process presently disclosedfeature a PAH concentration below 0.01% by weight, a Raman G/D ratiogreater than or equal to 2, a 2D/G ratio greater than or equal to 0.8(when measured using an incident laser with a wavelength of 514 nm) anda specific surface area (BET) of 250 m²/g or greater.

For example, the graphene nanosheets have a tap density of less thanabout 0.06 g/cm³, as measured by ASTM B527-15 standard.

For example, the graphene nanosheets have a tap density of less thanabout 0.04 g/cm³, as measured by ASTM B527-15 standard.

For example, the graphene nanosheets have a tap density of about 0.03 toabout 0.05 g/cm³, as measured by ASTM B527-15 standard.

For example, the graphene nanosheets have a tap density of about 0.03 toabout 0.04 g/cm³, as measured by ASTM B527-15 standard.

For example, the graphene nanosheets have a tap density of about 0.03g/cm³, as measured by ASTM B527-15 standard.

For example, the graphene nanosheets have a specific surface area(B.E.T) greater than about 250 m²/g.

For example, the graphene nanosheets have a specific surface area(B.E.T) greater than about 300 m²/g.

For example, the graphene nanosheets have a specific surface area(B.E.T) greater than about 350 m²/g.

For example, the graphene nanosheets have a specific surface area(B.E.T) of about 250 to about 600 m²/g.

For example, the graphene nanosheets have a specific surface area(B.E.T) of about 300 to about 600 m²/g.

For example, the graphene nanosheets have a specific surface area(B.E.T) of about 400 to about 600 m²/g.

For example, the graphene nanosheets have a specific surface area(B.E.T) of about 500 to about 600 m²/g.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 500 ppm.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 400 ppm.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 200 ppm.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 100 ppm.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 90 ppm.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 80 ppm.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 70 ppm.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 60 ppm.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 50 ppm.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 40 ppm.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration below 35 ppm.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 0.6% by weight.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 0.5% by weight.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 0.4% by weight.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 0.3% by weight.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 0.2% by weight.

For example, the graphene nanosheets have polyaromatic hydrocarbonconcentration of less than about 0.1% by weight.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 0.01% by weight.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of about 0.01% to about 0.7%.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of about 0.01% to about 0.5%.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of about 0.01% to about 0.3%.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of about 0.1% to less than about 0.3% by weight.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of about 0.01% to about 0.1%.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of about 0.15% to less than about 0.25% by weight.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of about 0.1% to about 0.6% by weight.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of about 0.05% to about 0.6% by weight.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of about 0.05% to about 0.5% by weight.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of about 0.1% to about 0.5% by weight.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of about 0.01% to about 0.4% by weight.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of about 0.05% to about 0.4% by weight.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of about 0.1% to about 0.4% by weight.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration of about 0.05% to about 0.3% by weight.

For example, the graphene nanosheets have a polyaromatic hydrocarbonconcentration below detection limit, as measured by gas chromatographymass spectrometry (GC/MS) or by Soxhlet extraction method according toASTM D7771-11.

For example, the graphene nanosheets have a Raman G/D ratio greater thanor equal to about 3 and a 2D/G ratio greater than or equal to about 0.8,as measured using an incident laser wavelength of 514 nm. Graphenenanosheets having a Raman G/D ratio greater than or equal to about 2.5and a 2D/G ratio greater than or equal to about 0.8, as measured usingan incident laser wavelength of 514 nm, wherein the graphene nanosheetshave a polyaromatic hydrocarbon concentration of less than about 0.7% byweight.

For example, the graphene nanosheets have a tap density of less thanabout 0.06 g/cm³, as measured by ASTM B527-15 standard.

For example, the graphene nanosheets are thermally produced.

For example, the volatile impurities are polyaromatic hydrocarbons.

For example, the reactive atmosphere is an oxidative atmosphere.

For example, the oxidative atmosphere comprises an oxidation agentchosen from air, water vapor, oxygen, ozone, peroxides, F₂, CO₂, H₂O,NO₂, Cl₂, alcohols, sulfuric acid, perchloric acid, persulfates acid,hypohalites, halogens, oxyhalides, nitrous oxides and mixtures thereof.

For example, the oxidative atmosphere comprises an inert gas and anoxidation agent.

For example, the inert gas is nitrogen, argon helium, neon, krypton,xenon or a mixture thereof.

For example, the gas mixture comprises oxygen and argon.

For example, the process comprises injecting a gas mixture comprisingoxygen into an enclosure containing the graphene nanosheets.

For example, the gas mixture is injected under constant flow.

For example, the gas mixture is injected under constant flow of about1-10 slpm.

For example, the reactive atmosphere is a reductive atmosphere.

For example, the reductive atmosphere comprises NH₄, H₂, H₂S, CO, andmixtures thereof.

For example, the reductive atmosphere comprises an inert gas and areducing agent.

For example, the process is effective for lowering a polyaromatichydrocarbon concentration below about 2% in the graphene nanosheets.

For example, the process is effective for lowering a polyaromatichydrocarbon concentration below about 1% in the graphene nanosheets.

For example, the graphene nanosheets are heated at a temperature of atleast about 300° C.

For example, the graphene nanosheets are heated at a temperature of atleast about 400° C.

For example, the graphene nanosheets are heated at a temperature of atleast about 500° C.

For example, the graphene nanosheets are heated at a temperature of atleast about 600° C.

For example, the graphene nanosheets are heated at a temperature ofabout 200° C. to about 1000° C.

For example, the graphene nanosheets are heated at a temperature ofabout 200° C. to about 750° C.

For example, the graphene nanosheets are heated at a temperature ofabout 300° C. to about 550° C.

For example, the process is carried out under atmospheric pressure.

For example, the process is carried out under below atmospheric pressureor under partial vacuum.

For example, the process is carried out under above atmosphericpressure.

For example, the specific surface area (B.E.T) is increased by at least20%.

For example, specific surface area (B.E.T) is increased by at least 30%.

For example, the specific surface area (B.E.T) is increased by at least40%.

For example, the specific surface area (B.E.T) is increased by at least50%.

For example, the specific surface area (B.E.T) is increased by at least60%.

For example, the specific surface area (B.E.T) is increased by at least70%.

For example, the specific surface area (B.E.T) is increased by at least80%.

For example, the specific surface area (B.E.T) is increased by at least90%.

For example, the specific surface area (B.E.T) is increased by at least100%.

For example, the process is carried out in the absence of a liquid orsolvent.

For example, the process is a dry process.

For example, the process is a continuous process.

For example, the process is carried out in a fluidized bed reactor.

For example, the process is carried out in a rotating oven.

For example, the process is a batch process.

For example, the tap density, as measured by ASTM B527-15 standard, ofthe graphene nanosheets remains substantially unchanged.

For example, the tap density of the graphene nanosheets is increased ordecreased by less than 5%, as measured by ASTM B527-15 standard.

For example, the tap density of the graphene nanosheets is increased ordecreased by less than 10%, as measured by ASTM B527-15 standard.

For example, the polyaromatic hydrocarbon is chosen from Biphenylene,Acenaphthylene, Phenanthrene, Anthracene, Fluoranthene, Pyrene, Xylenes,Napthalene, Benzo(A)Pyrene (BaP), Benzo[E]pyrene (BeP),Benzo[a]anthracene (BaA), Chrysen (CHR), Benzo[b]fluoranthene (BbFA),Benzo[j]fluoranthene (BjFA), Benzo[k]fluoranthene (BkFA),Dibenzo[a,h]anthracene (DBAhA), and mixtures thereof.

For example, the polyaromatic hydrocarbon is chosen from Biphenylene,Acenaphthylene, Phenanthrene, Anthracene, Fluoranthene, Pyrene, Xylenes,Napthalene, Benzo(A)Pyrene (BaP), Benzo[E]pyrene (BeP),Benzo[a]anthracene (BaA), Chrysen (CHR), Benzo[b]fluoranthene (BbFA),Benzo[j]fluoranthene (BjFA), Benzo[k]fluoranthene (BkFA),Dibenzo[a,h]anthracene (DBAhA), and mixtures thereof.

For example, the graphene nanosheets are quenched with a quench gashaving a temperature below 1300° C.

For example, the graphene nanosheets are quenched with a quench gashaving a temperature below 900° C.

For example, the graphene nanosheets are quenched with a quench gashaving a temperature below 600° C.

For example, the graphene nanosheets are quenched with a quench gashaving a temperature below 300° C.

For example, the graphene nanosheets are quenched with a quench gashaving a temperature below 100° C.

For example, the carbon-containing substance is injected at a quench gasto carbon ratio of at least 50 slpm of quench gas per mole of carbon perminute.

For example, the carbon-containing substance is injected at a quench gasto carbon ratio of at least 160 slpm of quench gas per mole of carbonper minute.

For example, the carbon-containing substance is injected at a quench gasto carbon ratio of at least 250 slpm of quench gas per mole of carbonper minute.

For example, the carbon-containing substance is injected at a quench gasto carbon ratio of about 50 slpm to about 125 slpm of quench gas permole of carbon per minute.

For example, the carbon-containing substance is injected at a quench gasto carbon ratio of about 100 slpm to about 250 slpm of the quench gasper mole of carbon per minute.

For example, the injecting of the carbon-containing substance beingcarried out using a plurality of jets.

For example, the injecting of the carbon-containing substance beingcarried out using at least 3 jets.

For example, the injecting of the carbon-containing substance beingcarried out using at least 4 jets.

For example, the injecting of the carbon-containing substance beingcarried out using at least 5 jets.

For example, the injecting of the carbon-containing substance beingcarried out using more than 5 jets.

For example, the graphene nanosheets are produced at a rate of at least120 g/h.

For example, the graphene nanosheets are produced at a rate of at least150 g/h.

For example, the graphene nanosheets are produced at a rate of at least200 g/h.

For example, the graphene nanosheets are produced at a rate of at least250 g/h.

For example, the graphene nanosheets are produced at a rate of about 120to about 150 g/h.

For example, the graphene nanosheets are produced at a rate of about 150to about 250 g/h.

For example, the graphene nanosheets are quenched with a quench gas fedat a rate of at least 3 slpm of the quench gas per kW of supplied torchpower.

For example, the graphene nanosheets are quenched with a quench gas fedat a rate of at least 1 slpm of the quench gas per kW of supplied torchpower.

For example, the graphene nanosheets are quenched with a quench gas fedat a rate of at least 0.5 slpm of the quench gas per kW of suppliedtorch power.

For example, the graphene nanosheets are quenched with a quench gas fedat a rate of about 0.5 slpm to about 1.5 slpm of the quench gas per kWof supplied torch power.

For example, the graphene nanosheets are quenched with a quench gas fedat a rate of about 1.5 slpm to about 4 slpm of the quench gas per kW ofsupplied torch power.

For example, the graphene nanosheets are produced at a rate of at least1 g/kWh of supplied plasma torch power.

For example, the graphene nanosheets are produced at a rate of at least2.5 g/kWh of supplied plasma torch power.

For example, the graphene nanosheets are produced at a rate of at least3 g/kWh of supplied plasma torch power.

For example, the graphene nanosheets are produced at a rate of at least5 g/kWh of supplied plasma torch power.

For example, the graphene nanosheets are produced at a rate of about 2to about 3 g/kWh of supplied plasma torch power.

For example, the graphene nanosheets are produced at a rate of about 3to about 5 g/kWh of supplied plasma torch power.

For example, the carbon-containing substance is a carbon-containing gas.

For example, carbon-containing gas is a C₁-C₄ hydrocarbon.

For example, the carbon-containing gas is chosen from methane, ethane,ethylene, acetylene, vinyl chloride propane, propene, cyclopropane,allene, propyne, butane, 2-methylpropane, 1-butene, 2-butene,2-methylpropene, cyclobutane, methylcyclopropane, 1-butyne, 2-butyne,cyclobutene, 1,2-butadiene, 1,3-butadiene or 1-buten-3-yne or a mixturethereof.

For example, the carbon-containing substance is a carbon-containingliquid.

For example, carbon-containing liquid is a C₅-C₁₀ hydrocarbon.

For example, the carbon-containing liquid is chosen from n-propanol,1,2-dichloroethane, allyl alcohol, propionaldehyde, vinyl bromide,pentane, hexane, cyclohexane, heptane, benzene, toluene, xylene orstyrene or mixtures thereof.

For example, the carbon-containing substance is methane or natural gas.

For example, the carbon-containing substance is a carbon-containingsolid.

For example, the carbon-containing solid is chosen from graphite, carbonblack, norbornylene, naphthalene, anthracene, phenanthrene,polyethylene, polypropylene, or polystyrene or mixtures thereof.

For example, the carbon-containing gas, carbon-containing liquid orcarbon-containing solid is in admixture with a carrier gas.

For example, the carrier gas comprises an inert gas.

For example, the inert gas is chosen from argon, helium, nitrogen,hydrogen or a mixture thereof.

For example, the quench gas is chosen from argon, helium, nitrogen,hydrogen or a mixture thereof.

For example, the quench gas comprises an inert gas.

For example, the quench gas comprises hydrogen.

For example, the quench gas comprises argon.

For example, the quench gas is fed at a rate of 1 to 10 slpm of gas foreach kW of supplied plasma torch power.

For example, the thermal zone has a temperature of about 4000° C. toabout 11 000° C.

For example, the thermal zone has a temperature of about 3000° C. toabout 8000° C.

For example, the thermal zone has a temperature of about 2600° C. toabout 5000° C.

For example, the carbon-containing substance is injected at a velocityof at least 70 m/s STP.

For example, the carbon-containing substance is injected at a velocityof at least 90 m/s STP.

For example, the carbon-containing substance is injected at a velocityof at least 100 m/s STP.

For example, the carbon-containing substance is injected at a velocityof about 60 to about 100 m/s STP.

For example, the carbon-containing substance is injected at a velocityof about 70 to about 90 m/s STP.

For example, the carbon-containing substance is injected at a velocityof about 75 to about 85 m/s STP.

For example, the process can be carried an injection angle of thecarbon-containing substance that is about 10 to about 40, about 20 toabout 30 degrees or about 25 degrees with respect to the axis ofsymmetry of the plasma.

For example, the process can be carried an injection angle of thecarbon-containing substance that is about 15 to about 35, about 20 toabout 30 degrees or about 25 degrees with respect to the axis ofsymmetry of the plasma.

For example, the process can be carried out using a plasma torchcomprising multi-hole injector for injecting the carbon-containingsubstance, wherein for each of injector holes, injection velocity is atleast 60 m/s STP and injection angle is about 15 to about 35 degreeswith respect to the axis of symmetry of the plasma.

For example, the process can be carried out using a plasma torchcomprising multi-hole injector for injecting the carbon-containingsubstance, wherein for each of injector holes, injection velocity is atleast 60 m/s STP and injection angle is about 20 to about 30 degreeswith respect to the axis of symmetry of the plasma.

For example, the process can be carried out using a plasma torchcomprising multi-hole injector for injecting the carbon-containingsubstance, wherein for each of injector holes, injection velocity is atleast 60 m/s STP and injection angle is about 25 degrees with respect tothe axis of symmetry of the plasma.

For example, the quench gas is injected around the thermal zone.

For example, the process further comprises collecting the producedgraphene nanosheets.

For example, the produced graphene nanosheets are collected in bagfilters, on filter cartridges or with a cyclone.

For example, the graphene nanosheets have a B.E.T. specific surface areagreater or equal than 250 m²/g as measured by ASTM D 3663-78.

For example, the graphene nanosheets have an aspect ratio of at least5:1.

For example, the graphene nanosheets have an aspect ratio of at least10:1.

For example, the graphene nanosheets have a Raman G/D ratio of at least3, as measured using an incident laser wavelength of 514 nm.

For example, the graphene nanosheets have a Raman 2D/G ratio of at least0.8, as measured using incident laser wavelength of 514 nm.

For example, the supplied plasma torch power is greater than 35 kW.

For example, the supplied plasma torch power is greater than 100 kW.

For example, the supplied plasma torch power is greater than 200 kW.

For example, the supplied plasma torch power is greater than 1000 kW.

The following examples are non-limitative and are used to betterexemplify the materials and processes of the present disclosure. Thescope of the claims should not be limited by specific embodiments andexamples provided in the disclosure, but should be given the broadestinterpretation consistent with the disclosure as a whole.

EXAMPLES Example 1: Thermal Treatment for Lowering PolyaromaticHydrocarbon Content of Graphene Nanosheets

In one exemplary embodiment, crumpled graphene nanosheets powderproduced using an ICP plasma torch using methane as precursor (asdescribed in U.S. provisional application No. 62/437,057) is treated ina dry process to remove the PAH. Prior to the gas-phase (dry) process,the produced crumpled graphene powder contains 0.16% wt. of PAH (asmeasured by Soxhlet extraction with toluene) and has a B.E.T. specificsurface area of 302 m²/g.

Referring now to FIG. 1A, the enclosure (oven) comprises a port (11) forinjection of a mixture of gases as well as a port (12) for exhaust of amixture of gases containing decomposed PAHs. Referring now to FIG. 1B,the enclosure comprises a flange (13) for containing both ports (12) and(13). Another port (14) is provided in the enclosure for inserting ashaft to mix the powder during the thermal oxidative treatment. Theenclosure is contained in walls (15), comprises a groove (16) foro-rings to seal the enclosure, as well as bottom (17) and side (18)heating elements. A person skilled in the art will appreciated that allother suitable enclosures may be used for carrying out the processesdisclosed herein.

The as-produced graphene nanosheets powder contains 0.16% wt. PAH (asmeasured by Soxhlet extraction with toluene) and has a B.E.T. specificsurface area of 302 m²/g. The gas-phase (dry) process to remove the PAHcomprises a thermal oxidative treatment under a constant massive gasflow (3 slpm) comprising of a mixture of Ar and O2 (9% vol. O2). Thetemperature inside the heated enclosure is approximately 400° C. and theenclosure is held at atmospheric pressure. The gas flow is used toensure that volatile components are removed and exhausted from thepowder. A 1 hour ramp is used to reach 400° C., followed by a plateau of40 minutes and approximately 2 hours of cooling. Between 10 g to 400 gwere treated per run. The batch process can easily be converted to acontinuous process, for example and without loss of generality, by usinga fluidized bed thermal reactor or by passing the material to be treatedthrough a rotating oven or through another heated zone using, forexample, a conveyor belt. The quantity to be treated can easily bescaled-up by increasing the size of the oven, and thus the heated zone.

The thermal oxidative treatment evaporates and/or decomposes PAH presenton the graphene nanosheets. The graphene nanosheets have a highgraphitization state (high level of crystallinity) and do not sufferfrom significant mass loss during this thermal treatment. The weightloss during the treatment is related to the removal and destruction ofPAH (thus to the initial concentration of the PAH). The followingoxidation agents could, for example, be used instead of oxygen: air,water vapor, carbon dioxide, etc. A person skilled in the art willreadily understand that the atmosphere used (composition of gas as wellas pressure) as well as the quantity of material to be treated willdetermine the temperature as well as the duration to be used.

Disordered carbon has a rate of etching one order of magnitude higherthan graphitized carbon. The tap density of the powder (as measured byASTM standard B527-15) is not modified by the thermal oxidativetreatment but an important increase in the B.E.T. specific surface areais measured (due to the removal of PAH blocking pores). A slightincrease in the concentration of oxygen functional groups on thegraphene surface can be observed (from approximatively 1 to 2% at. O/Cas measured by XPS). After the treatment, the resulting graphenenanosheets contain no more measurable PAH (under the 90 ppm detectionlimit of the Soxhlet extraction method). After the treatment describedin the present example, the B.E.T. specific surface area increased from302 m²/g prior to treatment to 567 m²/g post treatment. For graphenewith a higher Raman G/D ratio, we measured a change of B.E.T. specificsurface area from 300 to 450 m²/g, without significant mass loss.

Following the thermal oxidation treatment, there is an enhancement ofspecific B.E.T. specific surface area without measured weight loss(apart from the removed PAH fraction) and without change to the tapdensity. The increase in specific B.E.T. specific surface area istypically from about 20 to about 100%.

There are no noticeable changes in the relative ratios of the 3 mainRaman bands (D, G and 2D bands) on the Raman spectra of the as-producedgraphene as compared to the thermally treated material.

Example 2

In a second exemplary embodiment, a second type of oven is used toremove the PAH. The second oven used is a kiln furnace from Paragon withnatural convection; the flow of air (O₂/N₂, 21/79% by volume) is notforced inside but results from natural convection due to the high andlow temperature respectively inside and outside of the oven. Twocircular openings (with diameters of about one half inch) are located onthe side of the oven to allow natural convection to circulate and renewair in the oven. For this example, graphene nanosheets produced using anICP plasma torch using methane as precursor gas (as described inprovisional application 62/437,057) is treated in a gas-phase process toremove the PAH. For this example, the as-produced graphene contains0.50% wt. PAH (as measured by Soxhlet extraction with toluene) and has aB.E.T. specific surface area of 288 m²/g. The treatment is carried outat atmospheric pressure.

The temperature-time profile for this oven is as follows: from roomtemperature to 290° C. in 40 minutes, then from 290° C. to 440° C. in 20minutes, followed by a plateau of 2 hours in duration at 440° C. and aslow cooling to room temperature (lasting about 2 hours). The materialto be treated is positioned on multi-staged plates in the oven; eachplate contains from 40 to 80 grams of evenly layered graphene powder(from 160 to 320 grams are treated per batch). Again this batch processcan easily be converted to a continuous process.

After the treatment, the resulting graphene nanosheets contain no moremeasurable PAH (under the detection limit of the Soxhlet extractionmethod).and the B.E.T. specific surface area rises to 430 m²/g. The tapdensity of the powder (as measured by ASTM standard B527-15) is notmodified but an important increase in the B.E.T. specific surface areais measured, again without significant mass loss. A slight increase inthe concentration of oxygen functional groups on the graphene surfacecan be observed (from 1% to 2% approximately as measured by XPS).

There are no noticeable changes in the relative ratios of the 3 mainRaman bands (D, G and 2D bands) on the Raman spectra of the as-producedgraphene as compared to the thermally treated material.

Example 3: Counter Example

Using the same as produced crumpled graphene powder from example 1 (PAHconcentration of 0.16% wt. PAH, B.E.T. specific surface area of 302m²/g) and subjecting it directly (without the thermal oxidativetreatment) to Soxhlet extraction with toluene, the final B.E.T. specificsurface area was 329 m²/g. Knowing that the precision of the specificsurface area measurement is about ±10%, there is no significant changeobserved.

Following the Soxhlet extraction step (wet process), a change in the tapdensity of the crumpled graphene powder was observed. The tap densityincreased from 0.04 g/cm³ to 0.11 g/cm³. This supports that a wetprocess leads to an increase in tap density.

Subjecting this Soxhlet extracted sample (having a B.E.T. of 329 m²/g)to a subsequent heat treatment step in pure argon atmosphere (at about300° C. for 1.5 h) did not further increase the B.E.T. specific surfacearea. The further treated sample had a final B.E.T. specific surfacearea of 328 m²/g. We conclude that this heat treatment without thepresence of oxygen is not effective at increasing the specific surfacearea of the graphene, possibly by failing to remove the PAH from thegraphene's pores.

Example 4: Preparation of Graphene Nanosheets

The starting material (graphene nanosheets) of some processes disclosedin the present disclosure can be prepared in various different manners.For example, graphene nanosheets can be prepared by using a thermalplasma process as disclosed below.

In one exemplary embodiment, the hydrocarbon precursor material ismethane and it is injected into an inductively-coupled plasma torch(ICP) with a maximal plate power of 60 kW. FIG. 5 illustrates the ICPtorch 100 as well as the qualitative flow of the gases including thenon-carbon containing gases and the carbon containing substance.

For a power generator delivering 56 kW to an inductively coupled plasmatorch (PN-50 model, Tekna, Sherbrooke, Québec, Canada), and as shown inFIG. 5 , 20 slpm argon was used as central swirl gas 128, surrounded bya layer of quench gas (sheath gas) 124 consisting of 174 slpm of argonand 30 slpm of hydrogen gas. 33.6 slpm of natural gas (carbon feed gas)120 was injected through the injector probe with the designed nozzle110. Coils 122 conducting the radio frequency alternating currentgenerate the plasma. Qualitative isotherm lines 126 are shown inside theplasma torch. The pressure in the reactor was 500 torr. The injectionvelocity was 80.6 m/s at standard temperature and pressure (STP). It isto be understood that in the plasma state of extreme temperature andpressure, these gas injection velocities are greater and the value mustbe corrected to take the different temperature and pressure values intoconsideration. A person skilled in the art will understand that thisinjection velocity value will increase when the process is scaled, forexample for larger plasma volumes or larger plasma torch dimensions.

This process lasted 45 minutes and resulted in a graphene productionrate of 225 g/h as measured from the weight of powder harvesteddownstream of the hot plasma zone, divided by the operation time neededto synthesize this powder.

The carbon injected is 33.6 slpm/22.4 l=1.5 Mole/min or 18 g/min ofcarbon.

The quench gas to carbon ratio is at least 120 liters STP of non-carbongases to 1 Mole of carbon (also at least 180 slpm of non-carbon gases to18 g/min of carbon; 10.0 liters of non-carbon gas for 1 g of carbon ingas form).

The carbon injected per amount of power is typically 33.6 slpm for adelivered torch power of 56 kW which equals 0.6 slpm C/kW of torchpower.

Now referring to FIGS. 2A and 2B, the injector used is a multi-holenozzle 10 comprising five injection holes 12, each hole having a 0.052inch diameter. The nozzle 10 comprises a channel 16 for hydrocarbon feedand the surface of the nozzle 14 is perpendicular to the injection holes12. This configuration provides an injection velocity of 80.6 m/s STP.The carbon gas injection angle is 25 degrees with respect to the axis ofsymmetry of the plasma. A person skilled in the art will understand thata water-cooled injection nozzle will provide longer wear resistance andenable long duration production runs with stable operating conditions.

The resulting product was high quality graphene nanosheets, as seen fromthe Raman spectra (as shown in FIG. 3 ). The specific surface area ofthe material (using the B.E.T. method), once PAH are removed, is 431m²/g. The Raman spectrum of the product features a 2D/G ratio of 1.3 anda G/D ratio of 4.7 when measured using an incident wavelength of 514 nm.

The carbon precursor is injected at high velocity of at least 60 m/sSTP, typically 80 m/s STP, and even 100 m/s STP in order to limitresidence time in the hot zone. This may be achieved by injecting a gasmaterial, for example natural gas, through a showerhead-type nozzle withsmall holes, at an injection velocity that is greater than or equal tothe velocity of the plasma gas. A high feed rate coupled to small holesleads to a high injection velocity and a short residence time in the hotzone.

Example 5: Counter Example

Conversely, using similar parameters to those described above in Example4, but injecting the methane with an injection velocity below 60 m/s STPusing a single-hole nozzle, a significant fraction of carbon nodules andspheroidal carbon particles were produced leading to the typical Ramanspectrum of acetylene black (as shown in FIG. 4 ). FIG. 6 illustratesthe ICP torch 200 used in this counter example as well as thequalitative flow of the gases including the non-carbon containing gasesand the carbon containing substance.

In this example, and as shown in FIG. 6 , an injection velocity of 28.6m/s STP was used. The carbon precursor gas feed rate was 34.7 slpm CH4,and the achieved production rate was 142 g/h. 20 slpm argon is used ascentral swirl gas 228, surrounded by a layer of quench gas (sheath gas)224 consisting of 125 slpm of argon and 8 slpm of hydrogen gas.Otherwise the same method and apparatus were used as in Example 4. Thecarbon precursor gas 220 was injected through the injector probe withoutthe designed nozzle 210 (e.g. with a single-hole nozzle). Coils 222conducting the radio frequency alternating current generate the plasma.Qualitative isotherm lines 226 are shown inside the plasma torch.

The resulting material presents a low specific surface area (B.E.T.) of150 m²/g and a Raman spectra characteristic of thick graphitic nodulesinstead of thin graphenic particles (FIG. 4 ). The resulting particlesdisplay a Raman G/D ratio of 1.1 and a 2D/G ratio of 0.5 when measuredusing an incident wavelength of 514 nm. As illustrated in FIG. 6 , thecarbon precursor is injected into the hot zone via a single-hole probewithout a designed nozzle, thus leading to a longer residence time inthe hot zone, poor quenching efficiency and as a consequence theformation of acetylene-type carbon black (e.g. not graphene). The carbonprecursor gas is injected at an angle of zero degrees with respect tothe axis of symmetry of the plasma.

The embodiments of the paragraphs of the present disclosure arepresented in such a manner in the present disclosure so as todemonstrate that every combination of embodiments, when applicable canbe made. These embodiments have thus been presented in the descriptionin a manner equivalent to making dependent claims for all theembodiments that depend upon any of the preceding claims (covering thepreviously presented embodiments), thereby demonstrating that they canbe combined together in all possible manners. For example, all thepossible combinations, when applicable, between the embodiments of anyparagraphs and the processes and graphene nanosheets of THE SUMMARY arehereby covered by the present disclosure.

The scope of the claims should not be limited by specific embodimentsand examples provided in the disclosure, but should be given thebroadest interpretation consistent with the disclosure as a whole.

REFERENCES

-   1. Borm P J, et al., Formation of PAH-DNA adducts after in vivo and    vitro exposure of rats and lung cells to different commercial carbon    blacks, Toxicology and Applied Pharmacology, 2005 Jun. 1; 205(2):    157-167.-   2. Jeongmin Lim et al., A study of TiO₂/carbon black composition as    counter electrode materials for dye-sensitized solar cells.    Nanoscale Research Letters 2013; 8(1): 227.-   3. Stephen Brunauer, P. H. Emmett, Edward Teller, The Journal of the    American Chemical Society 60 (1938) 309.-   4. E. W. Lemmon and R. T Jacobsen, International Journal of    Thermophysics, Vol. 25 (2004) 21-68.

What is claimed is:
 1. Graphene nanosheets having a polyaromatichydrocarbon concentration of less than about 0.7% by weight, whereinsaid graphene nanosheets have a Raman G/D ratio greater than or equal toabout 2 and a 2D/G ratio greater than or equal to about 0.8, as measuredusing an incident laser wavelength of 514 nm.
 2. Graphene nanosheetshaving a polyaromatic hydrocarbon concentration of less than about 0.7%by weight.
 3. The graphene nanosheets of claim 2, wherein said graphenenanosheets have a tap density of less than about 0.04 g/cm³, as measuredby ASTM B527-15 standard.
 4. The graphene nanosheets of claim 2, whereinsaid graphene nanosheets have a polyaromatic hydrocarbon concentrationof less than about 500 ppm.
 5. The graphene nanosheets of claim 2,wherein said graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 200 ppm.
 6. The graphene nanosheets ofclaim 2, wherein said graphene nanosheets have a polyaromatichydrocarbon concentration of less than about 70 ppm.
 7. The graphenenanosheets of claim 2, wherein said graphene nanosheets have apolyaromatic hydrocarbon concentration of less than about 50 ppm.
 8. Thegraphene nanosheets of claim 2, wherein said graphene nanosheets have apolyaromatic hydrocarbon concentration below 35 ppm.
 9. Graphenenanosheets having a polyaromatic hydrocarbon concentration of about0.01% to about 0.5%.
 10. The graphene nanosheets of claim 9, whereinsaid graphene nanosheets have a tap density of less than about 0.06g/cm³, as measured by ASTM B527-15 standard.
 11. The graphene nanosheetsof claim 9, wherein said graphene nanosheets have a tap density of about0.03 to about 0.05 g/cm³, as measured by ASTM B527-15 standard.
 12. Thegraphene nanosheets of claim 9, wherein said graphene nanosheets have atap density of about 0.03 to about 0.04 g/cm³, as measured by ASTMB527-15 standard.
 13. The graphene nanosheets of claim 9, wherein saidgraphene nanosheets have a tap density of about 0.03 g/cm³, as measuredby ASTM B527-15 standard.
 14. The graphene nanosheets of claim 9,wherein said graphene nanosheets have a specific surface area (B.E.T)greater than about 250 m²/g.
 15. The graphene nanosheets of claim 9,wherein said graphene nanosheets have a specific surface area (B.E.T) ofabout 250 to about 600 m²/g.
 16. The graphene nanosheets of claim 9,wherein said graphene nanosheets have a specific surface area (B.E.T) ofabout 400 to about 600 m²/g.
 17. The graphene nanosheets of claim 9,wherein said graphene nanosheets have a polyaromatic hydrocarbonconcentration of less than about 0.3% by weight.
 18. The graphenenanosheets of claim 9, wherein said graphene nanosheets have apolyaromatic hydrocarbon concentration of less than about 0.2% byweight.
 19. The graphene nanosheets of claim 9, wherein said graphenenanosheets have polyaromatic hydrocarbon concentration of less thanabout 0.1% by weight.
 20. The graphene nanosheets of claim 9, whereinsaid graphene nanosheets have a Raman G/D ratio greater than or equal toabout 3 and a 2D/G ratio greater than or equal to about 0.8, as measuredusing an incident laser wavelength of 514 nm. Graphene nanosheets havinga Raman G/D ratio greater than or equal to about 2.5 and a 2D/G ratiogreater than or equal to about 0.8, as measured using an incident laserwavelength of 514 nm, wherein said graphene nanosheets have apolyaromatic hydrocarbon concentration of less than about 0.7% byweight.